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24 Cards in this Set
- Front
- Back
What proteins are thought to be involved with paracellular transport?
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Paracellins
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Compare filtered load and excreted load in reabsorption and in secretion.
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*reabsorption: filtered load > excreted load
*secretion: excreted load > filtered load |
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Why is movement of water from the interstitium to the peritubular capillaries favored?
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The oncotic pressure in the capillaries exceeds hydrostatic pressure, thus movement into the capillary is favored. Also, high flow aids in uptake.
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What tubular segment does not show transcellular reabsorption of NaCl?
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The descending limb of the loop of Henle.
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Is movement of Na+ from the lumen across the apical membrane with or against the electrochemical gradient? What is the benefit of this?
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It is a passive process that follows the electrochemical gradient, and therefore it can be coupled to drive other secondary transport mechanisms (Glc, H+, Cl-, etc.).
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Is movement of Na+ across the basolateral membrane into the interstitium a passive or active process?
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It is active and requires a carrier, usually Na+/K+ ATPase.
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Is paracellular movement of Na+ a passive or active process?
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It is usually passive.
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What two segments of the nephron feature an especially leaky epithelia? Compare the paracellular movement in these two segments.
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The proximal convoluted tubule (reabsorption) and descending loop of Henle (secretion of Na+).
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Which nephron segments feature a tight epithelia? Describe the movement of water through these segments.
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Ascending limb of the loop of Henle, distal tubule, and the collecting ducts. Water movement is transcellular and through aquaporins other than AQP-1.
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Describe AQP-1: where is it found? How is it controlled?
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AQP-1 is found in the PCT and vasa recta...it is constitutively expressed and not regulated by ADH.
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Describe the transport of water across the distal tubule and collecting ducts.
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In these segments water moves transcellular through AQP-2 which is regulated by ADH.
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Where is AQP-3 found? How is it regulated?
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AQP-3 is found on the basolateral membrane of distal tubule and collecting duct epithelia. It is non-regulated.
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Describe the osmolarity of the reabsorbate and the exiting filtrate of the proximal tubule.
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Because the proximal tubule reabsorbs large amounts of both water and solute, both the reabsorbate and the exiting fluid is isosmolar to plasma.
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Does the loop of Henle reabsorb more water or more solute? Describe the osmolarity of the reabsorbate and the exiting filtrate of this segment.
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The loop of Henle reabsorbs more solute than water. As a result, the reabsorbate is hyperosmolar to plasma, whereas the exiting filtrate is hyposmolar.
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Do the distal tubule and collecting ducts reabsorb more water or more solute? What determines this?
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These segments reabsorb more water than solute, but only when ADH is present. In its absence the converse is true.
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Describe the osmolarity of the reabsorbate and the exiting filtrate of the distal tubule and collecting ducts when ADH is present.
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Because more water than solute is reabsorbed in this situation, the reabsorbate is hyposmolar to plasma and exiting filtrate is hyperosmolar to plasma.
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Name three solutes that are co-transported with the reabsorption of Na+ in the proximal convoluted tubule.
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Reabsorption of Cl-, glucose, and amino acids are coupled to the passive uptake of Na+.
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What is the maximum activity for the Na+/glucose transporter in the proximal tubule?
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180mg/dL.
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How is Na+ moved across the basolateral membrane in the PCT? How is it then removed from the interstitium?
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Transport across the basolateral membrane is an active process facilitated by Na+/K+ ATPase. From the interstitium, Na+ is carried with water into the peritubular capillaries. Also there is some paracellular leakage back into the lumen.
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In the PCT there are open channels for water, but what powers its movement?
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Water moves down small, local, transient osmotic gradients.
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What is osmotic diuresis? How does it occur in diabetes?
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Failure to reabsorb an osmotic particle causes osmotic diuresis by creating an osmotic pressure that draws fluid into the lumen. In DM, glucose acts as this particle.
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Describe the movement of H+, bicarbonate, and CO2 across the apical membrane of the proximal tubule.
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H+ is secreted into the lumen via a Na+/H+ antiporter. It combines with bicarbonate (which does not pass the membrane on its own) to form CO2 and water. CO2 diffuses rapidly across the apical membrane into the cell.
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How is bicarbonate moved across the basolateral membrane?
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Movement is via a Cl-/bicarbonate ATPase which moves HCO3- out and Cl- in.
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What percentage of filtered urea is reabsorbed by the proximal tubule? How is this regulated?
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The proximal tubule reabsorbs 50% of filtered urea via unregulated passive diffusion.
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